The Gene Targeting Shared Resource was established 10 years ago to assist Cancer Center members who use animal models in constructing and managing genetically altered mouse strains. Located in the Woodbury Animal Shared Resource, this Shared Resource provides services including the generation of transgenic mice by pronuclear injection of DNA, gene targeting in mouse embryonic stem (ES) cells, generation of ES cell chimeras with genetically altered ES cells, and cryopreservation services for both embryos and sperm. To expand on these services further, the facility has recently developed protocols for producing ES cell-derived mice by tetraploid blastocyst injections, a procedure that has greatly facilitated the rapid generation of cancer mouse models at CSHL. It also generated ES cell lines for gene targeting in C57BL/6J mice, which, when fully incorporated, will enable researchers to produce pure mouse strains rapidly in one of the most widely used genetic backgrounds. Finally the Gene Targeting Shared Resource has added karyotyping as a step in the production of chimeric mice; by identifying ES cells with a normal chromosome complement, the efficiency of germline transmission has been greatly enhanced. The rapid incorporation of new technologies within the Resource helps keep research in the Cancer Center at the cutting edge. In addition, its internal development efforts continue to provide Cancer Center investigators with new tools and method to speed up the production of mouse models while maintaining low costs.